Prepacked well screens are widely used in the oil industry for sand control. In the course of completing an oil well, it is common to first run a string of casing into the wellbore and then run the production tubing inside the casing. At the site of the producing formation, the casing is perforated across one or more production zones to allow hydrocarbons to enter the wellbore. After the well is completed and placed in production, formation sand from unconsolidated formations may be swept into the flow path along with formation fluid. This sand is usually fine and can erode production components in the flow path. As one method for controlling sand production one or more sand screens or slotted liners are installed in the flow path inside the perforated casing, along with a gravel pack. A packer may be set above and/or below the sand screen to seal off the annulus in the producing zone from non-producing formations. Another common method is to place a screen and gravel pack in a producing interval without the cemented, perforated casing. This is often called an open hole gravel pack.
In general practice, the annulus around the screen in unconsolidated formations is packed with a relatively coarse sand or gravel which acts as a filter to reduce the amount of fine formation sand reaching the screen. To spot the gravel around the screen, a pump, a work string, and service seal unit are often used. During completion, this equipment is employed to pump gravel through the work string where the gravel is squeezed into the perforations in the producing formation and placed between the screen or slotted liner and well casing in a cased well, or into the annular space between the formation and the screen or slotted liner in an open hole completion. The gravel is pumped down the work string in a slurry of water or gel. Some spaces may remain that are not filled with gravel, and they can eventually fill up with accumulated formation sand, which is prevented from entering the wellbore only by the screen or slotted liner. Additionally, fine formation sand finding its way through the gravel packs in the production flowpath can cause screen erosion and failure, unless stopped by the screen. Thus, screens and slotted liners are used to help prevent formation sand from finding its way into production.
During the initial production period following the gravel packing operation, fine sand may be carried into or through the gravel pack before the gravel pack bridge stabilizes and yields clean production. Those fines tend to migrate through the gravel pack and screen or slotted liner. In some instances, this can cause severe erosion of the screen or slotted liner and ultimate failure of the screen or slotted liner to reduce sand invasion. In other situations, the sand fines may include plugging materials which are carbonaceous, siliceous or organic solids which can completely plug the screen or slotted liner flow passages and terminate production shortly after completion. In deep wells, when the screen or slotted liner becomes plugged and the internal pressure in the production tubing is reduced, the formation pressure can collapse the screen or slotted liner. In addition, when a substantial amount of sand has been lost from the surrounding formation, the formation may collapse with resultant damage to the well casing, liner, and/or screen.
Prepacked sand screens can be used in conjunction with an external gravel pack, as additional protection against the problems just described, or they can be used instead of an external gravel pack, as an alternative remedy. Prepacked sand screens essentially consist of a combination of wire screen and perforated casing with the annular space between filled with consolidated gravel, the theory being what one does not catch, the other will. The wire wrap is often on the outside as shown in FIG. 1. The prepacked screen can also be reversed, with the wire wrap on the inside and perforated casing or sleeve on the outside, as shown in FIG. 2.
More specifically, prepacked sand screens employ a perforated mandrel which is surrounded by longitudinally extending spacer bars, rods, or ribs over which a continuous wire is wrapped in a carefully spaced spiral configuration to provide a predetermined axial gap between the wire turns. The predetermined aperture between turns permits formation fluids to flow through the screen, while the closely spaced wire turns help exclude fine particulate material such as formation sand or fines.
The part of the prepacked screen of particular interest here comprises a uniform consolidated annulus of resin-coated sand or gravel filling a space between either a pair of concentric screen members, or a screen and a perforated liner.
Prepacked screens are especially useful in problem wells where it is difficult or impossible to complete the well in the producing zone by conventional techniques of running casing, cementing, down-hole perforating, running the screen jacket assembly, and then gravel packing with a cross-over tool.
The resin-coated gravel or sand used in these prepack screen assemblies is often of the type described in an article entitled, "Performance Review of Phenolic-Resin Gravel Packing" by Lowell W. Saunders and Howard L. McKinzie which appeared in the Feb. 1981 issue of "JOURNAL OF PETROLEUM TECHNOLOGY" at pages 221-228. Such gravel has been supplied as part of a precured packing between two concentric well screens, as well as used as a direct replacement for ordinary gravel packs which are placed behind slotted liners or screens. The gravel in the prepacked screen is cured after installation by temperature and/or catalyst. The cured resin-coated gravel is degradable by some acids and in strongly basic fluids having a high pH.
There are major problems associated with prepacked screens. In some situations where damage to the well has occurred, one option for cleaning up the well is the use of strong acids; however if a prepacked screen has been installed it precludes the use of harsh acids, which are needed to remove the damage, because the acid would destroy the coatings currently used on the gravel in the prepacked screen. The coating will depolymerize, and possibly break into chunks of material which will cause further wellbore damage. Therefore, when unexpected damage is incurred in the well, the presence of a prepacked screen makes it impossible to use acid to clean up the well.
When used, such assemblies are not easily removed in the event of a failure of the internal annular gravel pack, since the prepacked screens are typically made of stainless steel which is difficult to cut through. Furthermore, they often tear apart and jam as the wire and rod of which they are made comes apart.
Other operating conditions and chemicals commonly used in oil field operations will also destroy the currently used resin on the gravel. Another example is the use of high temperature steam for enhanced recovery. If this procedure is anticipated for the well, the use of a prepacked well screen is currently precluded.
U.S. Pat. Nos. 5,005,648, 4,842,072 and 4,800,960, incorporated by reference herein in their entirety, describe a resin system comprising a furan resin, a hydrolyzable diluent, such as butyl acetate, and an acid. This resin system can withstand high temperatures and acids. These references do not address any application to well screens.
U.S. Pat. No. 4,487,259 discloses a prepacked well screen and casing assembly adapted to be lowered into a well. The invention relates to the structure rather than composition and is intended to permit the well to be completed without the use of a down-hole gravel packing operation.
In U.S. Pat. No. 4,856,590 there is disclosed a process for washing through filter media in a production zone with a pre-packed screen and coil tubing. The object is to provide a process whereby a wash down of the bottom-hole assembly may be undertaken at a great savings and in a reduced amount of time.
U.S. Pat. No. 5,377,750 discloses a sand screen completion method. U.S. Pat. 5,232,048 discloses a well screen with an increased outer surface area. In U.S. Pat. No. 5,339,895 there is disclosed a prepacked well screen assembly for separating particulate material from formation fluid. None of these patents address the problem of finding an improved resin coating for the gravel in the prepacked screen which would be inert and withstand high temperatures.
In an SPE publication, SAND CONTROL, Vol. 1, Henry L. Doherty Series, Ch. 11, 1992, there is an article titled "PLASTIC CONSOLIDATION PRINCIPLES", by W. L. Penberthy, Jr., and C. M. Chaughnessy, in which it is stated that the upper temperature limit of resins used for sand control is about 300.degree. F. In the same publication, it is stated that furans or phenolic furans are very reactive . . . and no method is available to control them internally, such as, for example, in a deep well. This would appear to summarize the prevalent view of the art in this field. The consensus would seem to be that resin-coated sand or gravel which could withstand high temperatures and strong acids would make it much easier and less expensive to employ enhanced recovery with high temperature steam or to use strong acids for cleanup; and, further, that furan resins might have much potential for this kind of application, but previous formulations are unmanageable above approximately 300.degree. F.
There appears to be a need in the art for a prepacked well screen which could stand up to any harsh conditions encountered in oilfield operations.
There is a need in the art for a resin system for coating gravel packs in prepacked well screens which would make the screens inert to strong acids used for cleanup and to high temperature steam used in secondary recovery.